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Definition of breakdown types Type I: small transverse kick, affects only collision effect

H.H. Braun, Structure WG, 13.9.2007 Permissible Trip Rates for 12 GHz structures in CLIC Type of break downs

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Definition of breakdown types Type I: small transverse kick, affects only collision effect

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  1. H.H. Braun, Structure WG, 13.9.2007 • Permissible Trip Rates for 12 GHz structures in CLIC • Type of break downs • Permissible rate for type I RF breakdown • Permissible rate for type II RF breakdown • Permissible rate for type III RF breakdown • Conclusions

  2. Definition of breakdown types Type I: small transverse kick, affects only collision effect Type II: medium transverse kick, affects luminosity Type III: large transverse kick, destruction of main linac components

  3. Permissible Trip Rate for type I breakdown as a Function of Linac Energy Overhead EA EB beam loading T2field recovery T1pump down This requires that PETS can be switched off from one pulse to next and that field can be ramped with constant phase !

  4. If X is the probability that a structure break down in a pulse, we will have in average with N the total number of structures and M the number of structures connected to a PETS. Since NTrip is a random number it will scatter around this value with a standard deviation NTrip½ . Assuming that we want to cope with 6 standard deviations from <NTrip> we get the condition with R the fractional voltage reserve of the linac

  5. With L=0.229 m EA=100 MV/m EB=-20.5 MV/m N=3.3 TeV/(L EA)=144106 M=2 T1=20 s T2=20 s

  6. Permissible trip rate for type II RF breakdown The vertical momentum of the beam electrons have a gaussian distribution with this r.m.s. value varies along the linac in the range 5-35 keV/c Each single cell of an accelerating structure increases PZ by 954 keV/c. A change of field direction in a single cell by 0.30 during a breakdown event is therefore sufficient to bring the beams out of collisions during this pulse. If a field distortion of this magnitude occurs, the implication is that all machine pulses with a break-down in a single structure are lost for luminosity. If this is true the permissible breakdown probability X for a 1% luminosity loss is corresponding to a trip probability of X< 710-8 This reasoning is somewhat pessimistic, since the impact of a transverse kick depends on the local b function and the (random) azimuthal direction of the kick.

  7. Type III RF breakdown The geometric acceptance of the main linac is given by a transverse kick with can be sufficient to steer the beam on the structure aperture. To get 4 MV transverse kick 4 cells have to turn field by 900.

  8. Conclusions • Permissible rate for type I RF breakdown (with 5% voltage overhead) 10-5 • Permissible rate for type II RF breakdown 10-7structures at low energy are more critical than those at high energy • Permissible rate for type III RF breakdown 0 structures at low energy are more critical than those at high energy TBTS experiment essential to connect breakdown probability with DPT distribution of breakdowns !

  9. How important is dark current ?

  10. Why bother about dark current, overfocusing of main beam quadrupoles will clean off d.c. electrons ! But at high energy end of linac quadrupoles are spaced by 10 modules, or 80 accelerating structures.

  11. What’s a good value for EMAX? P-Linac people use Kilpatrick criterion for structure design with braveness factor. Braveness 2.5 is considered as pushing to the very limits. CTF II, Cu HIGGS Dolgatchev data RF gun’s

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